Tailoring expansion properties of balloons for medical devices

ABSTRACT

Dilatation balloons are fabricated of a biaxially orientable material such as a nylon or a polyamide material, and they have an inflated, non-distended working profile as well as a stretched inflated profile which is achieved by applying pressure through a dilatation catheter or the like that is in excess of that needed to achieve the inflated, non-distended profile and which is adequate to effect dilatation or the like up to a maximum pre-bursting pressure application. The maximum pre-bursting size of the balloon can be tailored depending upon the needs of the particular balloon within a wide range of possible maximum pre-bursting sizes.

This is a divisional of copending application Ser. No. 452,713, filedDec. 19, 1989, now U.S. Pat. No. 5,108,415 which is a divisional ofapplication Ser. No. 384,723, filed Jul. 24, 1989 (now U.S. Pat. No.4,906,244), which is a continuation-in-part of application Ser. No.253,069, filed Oct. 4, 1988. Now abandoned.

BACKGROUND AND DESCRIPTION OF THE INVENTION

The present invention generally relates to balloons for medical devices,as well as to processes and equipment for forming same. Moreparticularly, the invention relates to medical or surgical balloons andto catheters incorporating them, such as those designed for angioplasty,valvuloplasty and urological uses and the like. The balloons exhibit theability to be tailored to have expansion properties which are desiredfor a particular end use. The expansion property of particular interestis the amount or percentage of expansion or stretching beyond anon-distended inflated size at which the balloons are inflated to removefolding wrinkles but are not stretched. Balloons which are especiallysuitable in this regard are made of nylon or polyamide tubing that hasbeen biaxially oriented into the desired balloon configuration.

Catheter balloons and medical devices incorporating them are well-knownfor use in surgical contexts such as angioplasty procedures and othermedical procedures during which narrowings or obstructions in bloodvessels and other body passageways are altered in order to increaseblood flow through the obstructed area of the blood vessel. For example,in a typical balloon angioplasty procedure, a partially occluded bloodvessel lumen is enlarged through the use of a balloon catheter that ispassed percutaneously by way of the arterial system to the site of thevascular obstruction. The balloon is then inflated to dilate the vessellumen at the site of the obstruction.

Essentially, a balloon catheter is a thin, flexible length of tubinghaving a small inflatable balloon at a desired location along itslength, such as at or near its tip. Balloon catheters are designed to beinserted into a body passageway such as the lumen of a blood vessel, aheart passageway, a urological passageway and the like, typically withfluoroscopic guidance.

In the past, medical device balloon materials have included balloonshaving a wall thickness at which the material exhibits strength andflexibility that allow inflation to a working diameter or designatedinitial dilation diameter which, once achieved, is not surpassable toany significant degree without balloon breakage or substantiallyincreasing the risk of balloon breakage. Balloons of these materials canbe characterized as being substantially non-distensible balloons thatare not stretchable, expandable or compliant to a substantial extentbeyond this working diameter. Such substantially non-distensibleballoons can be characterized as being somewhat in the nature of papersbags which, once inflated to generally remove folding wrinkles, do notfurther inflate to any significant degree. Polymeric materials of thissubstantially non-distensible type that are used or proposed for use asmedical balloons include polyethylene terephthalates.

Other types of materials, such as polyvinyl chlorides and cross-linkedpolyethylenes can be characterized as being distensible in that theygrow in volume or stretch with increasing pressure until they break.These materials are generally elastic and/or stretchable. When suchextensible materials are used as medical balloons, the working diameteror designated dilation diameter of the balloon can be exceeded, basedupon the stretchability of the material.

Substantially non-distensible balloons have at times been considered tobe advantageous because they will not inflate significantly beyond theirrespective designated dilation diameters, thereby minimizing possibleover-inflation errors. The theory is that one need only select a balloonsuch as an angiographic balloon that has an opened and inflated diameterwhich substantially corresponds to the dilation size desired at theobstruction site. However, physiological vessels such as arteries aregenerally tapered and do not always coincide with readily availablecatheter balloon dimensions, and at times it may be preferable to beable to increase the diameter of the balloon beyond what had beeninitially anticipated. With a substantially non-distensible balloon,such further extension is severely limited. On the other hand, certainnon-distensible materials out of which medical balloons are madegenerally possess relatively high tensile strength values, which istypically a desirable attribute, especially for dilating tough lesions.

More readily distensible materials such as polyvinyl chloride typicallyexhibit a lower tensile strength and a larger elongation than asubstantially non-distensible material such as polyethyleneterephthalate. This relatively low tensile strength increases the riskof possible balloon failure. Due to their larger ultimate elongation,most readily distensible materials can provide a wide range of effectiveinflation or dilation diameters for each particular balloon size becausethe inflated working profile of the balloon, once achieved, can befurther expanded in order to effect additional dilation. But this veryproperty of having an expanded dilation range is not without its dangersbecause of the increased risk of overinflation that can damage the bloodvessel being treated, and the overinflation risk may have to becompensated for by using balloon wall thicknesses greater than mightotherwise be desired.

Although a material such as polyethylene terephthalate is advantageousfrom the point of view of its especially high tensile strength and itstightly controllable inflation characteristics, it has undesirableproperties in addition to its general non-distensibility. In somesituations, biaxial orientation of polyethylene terephthalate willimpart excessive crystallinity to an angioplasty balloon, or the Young'smodulus will be simply too high. Under these circumstances, the balloonitself or the thicker leg sections thereof will not readily fold overand down in order to provide the type of low profile that is desirablefor insertion through a guiding catheter and/or through thecardiovascular system and especially through the narrowed artery whichis to be dilated. The resistance to folding, or "winging," is anespecially difficult problem when it comes to larger balloons, such asthose intended for valvuloplasty applications.

Also, it has been observed that thin-walled materials such aspolyethylene terephthalate have a tendency to form pin holes or exhibitother signs of weakening, especially when flexed. Such a tendency canrequire extreme care in handling so as to avoid inadvertent damage thatcould substantially weaken a polyethylene terephthalate medical balloon.Although it is known that a lower profile and more flexible balloon andballoon legs can be made by thinning the wall, thus thinned polyethyleneterephthalate balloons become extremely fragile and may not surviveinsertion through the cardiovascular system with desired integrity andwithout pin-holing and/or rupture.

Materials such as polyethylene terephthalate do not readily acceptcoating with drugs or lubricants, which can be desirable in manyapplications. Polyethylene terephthalate materials are also difficult tofuse, whether by adherence by heating or with known biocompatibleadhesives.

By the present invention, these undesirable and/or disadvantageousproperties of substantially non-distensible medical balloons such asthose made from polyethylene terephthalate materials are significantlyeliminated. At the same time, many of the advantages of these types ofmaterials are provided or approximated. Furthermore, the presentinvention realizes many of the advantages of the more elastomericmaterials such as polyvinyl chloride, but without the disadvantages ofrelatively low tensile strength, and the possibility of excessiveexpandability that can lead to overinflation of a blood vessel or thelike.

In summary, the present invention achieves these objectives and providesadvantageous properties along these lines by forming and providing acatheter and medical catheter balloon constructed of a material that isof limited and generally controlled distensibility whereby expansionbeyond the working or fully expanded but non-distended dilation profileof the balloon is possible, but only to a desired extent which can,within limits, be tailored to the particular needs of the balloondevice. The invention includes utilizing a tailorable material such as anylon material or a polyamide material that is formed into a biaxiallyoriented balloon by appropriate axial elongation, radial expansion andheat treatment procedures.

It is a general object of the present invention to provide an improvedmedical balloon and medical device incorporating a balloon.

Another object of the present invention is to provide an improvedmedical balloon that exhibits a controlled distensibility at highpressure.

Another object of the present invention is to provide an improvedballoon and/or catheter and method of making same which provides a rangeof dilation capabilities while minimizing risks that could be associatedwith such flexibility.

Another object of the present invention is to provide an improvedangioplasty catheter balloon molding method and apparatus that areespecially suitable for forming tubing of tailorable materials such asnylon or polyamide into catheter balloons.

Another object of the present invention is to provide an improved meansand method for biaxially orienting relatively amorphorous tubing intobiaxially oriented medical balloons.

Another object of the present invention is to provide an improvedballoon catheter that is readily coated with materials such aslubricating agents and the like which are advantageous foradministration in association with balloon catheters.

Another object of this invention is to provide an improved balloon thatexhibits a combination of good strength and desirable flexibility.

Another object of this invention is to provide a medical balloon thatexhibits an ability to be readily fused to other components.

These and other objects, features and advantages of this invention willbe clearly understood through a consideration of the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of this description, reference will be made to theattached drawings, wherein:

FIG. 1 is an elevational illustration, partially in cross-section, of aballoon catheter having a structure typical of that suitable forangioplasty;

FIGS. 2a, 2b, 2c, 2d and 2e are elevational illustrations of tubing asit is progressively processed to transform same into a medical balloonaccording to this invention;

FIG. 3 is a cross-sectional view illustrating a preferred apparatus forcarrying out the processing steps illustrated in FIGS. 2a, 2b, 2c, 2dand 2e;

FIG. 4 is a plot of balloon size versus balloon inflation pressure forballoons made of a variety of materials, including nylon balloonstailored to provide different radial expansion properties;

FIGS. 5 and 6 are plots illustrating the function of heat setting ontailorability; and

FIG. 7 is a plot illustrating the function of hoop expansion ratio ontailorability.

DESCRIPTION OF THE PARTICULAR EMBODIMENTS

An illustrative catheter is generally designated in FIG. 1 by referencenumeral 21. Catheter 21 includes a catheter tube 22 having a proximalend 23 and a distal end 24. A guiding catheter 25 is also illustrated. Amedical balloon 26 is shown secured to the distal portion of thecatheter tube 22 in a location overlying one or more apertures 27through the catheter tube. Extending through the lumen of the cathetertube 22 is an inner elongated body 28, such having a proximal end 29 anda distal tip end 31. The inner body 28 may be solid or have an internallumen, depending upon the function that the inner body is to perform,whether it be simply a guiding function or whether it is intended toalso provide the capability to insert materials into the bloodstream ormeasure parameters of the bloodstream, or the like.

Except for the balloon 26, all of these various components performfunctions that are generally appreciated in the art. Typically with theaid of the guiding catheter 25, the inner body 28 and the catheter tube22 are inserted into the cardiovascular system until the balloon islocated at an occlusion site. At this stage, the balloon 26 is typicallyfolded and collapsed, and it has an external diameter less than theinflated diameter illustrated in FIG. 1, to the extent that the balloon26 is generally wrapped around the catheter tube 22. Once the balloon 26is maneuvered to the location of the occlusion, a pressurized fluid isinserted at the proximal end 23 of the catheter tube 22 for passagethrough aperture 27 and for inflation of the balloon 26. This unfoldsthe balloon until it presents a relatively smooth outer surface orworking profile for imparting forces that are radially outwardlydirected at the desired site within the body in order to achieve thedesired result of lesion dilation, occlusion reduction or similartreatment. In accordance with an important aspect of the invention, thepassage of greater pressure against the inside surface of the balloon 26permits the outer surface of the balloon 26 to transmit additional forceto the lesion or the like, which may include actual further radiallyoutwardly directed movement of the balloon 26 beyond its workingprofile, as illustrated in phantom in FIG. 1.

FIGS. 2a, 2b, 2c, 2d and 2e illustrate the transformation of a length oftubing 32 that is a material such as extruded nylon or polyamide into abiaxially oriented balloon 33 or a modified biaxially oriented balloon34. Either balloon 33 or 34 possesses the ability to be first inflatedto its unextended or working profile and then therebeyond to a limitedand/or controlled extent by the application of greater pressure. Eachballoon 33 and 34 includes a balloon portion 35, 36, respectively, andleg portions 37, 38, respectively. The leg portions 37, 38 are theportions of the balloon that are secured to the tube 22 of the catheter21.

In an exemplary application, tubing length 32 is extruded so that itexhibits a diameter that is roughly one-quarter of the diameter intendedfor the balloon. The extruded tubing length 32 also has a nominal wallthickness that is on the order of six to twelve or so times the desiredwall thickness of the balloon 33, 34.

FIG. 2b illustrates tubing length 32a, which is tubing length 32, afterit has been axially oriented or elongated to approximately three timesits original length. This elongation or drawing procedure is carried outat approximately room temperature, and same proceeds typically until ithas been stretched to the point that it exhibits a noticeable resistanceto further stretching. Typically, the pull force is greater than theyield point of the particular tubing length 32, but less than theultimate tensile strength, lengthwise, of the selected material.Generally speaking, this axial elongation procedure is carried out untilthe wall thickness of the tubing length 32a is roughly one-half of thewall thickness of the tubing length 32 and/or until the diameter of thetubing length 32a is roughly one-half to forty percent of the outerdiameter of the tubing length 32. The actual stretched length cantypically be about two times to about four times or more of the originaltubing length 32. Actual stretching of the tubing 32 can be performed bysimply axially stretching length 32 or by pulling or drawing length 32through a sizing die.

FIG. 2c illustrates a step that is carried out after the longitudinalorientation procedure of FIG. 2b has been completed. This is a biaxialorientation step by which a portion of the tubing length 32a expandsprimarily radially and thereby delineates the balloon portion 35 fromthe leg portions 37. This biaxial orientation is carried out by pressureexerted on the inside wall of the tubing length 32a by a pressurizedfluid. Typically, the balloon portion 35 will have an outer diameterthat is on the order of roughly six times the outer diameter of thetubing length 32a. The pressurized fluid may include gasses such ascompound air, nitrogen or argon. Liquids such as water or alcohol couldalso be used if they do not pose a problem of leaving residual fluid inthe balloon. Generally speaking, the larger the balloon, the faster willbe the inflation fluid flow rate. Examples of cardiac balloons generallyranked in typical order of increasing size are those designed for use incoronary arteries, those designed for use in peripheral arteries, andvalvuloplasty balloons for use in cardiac valves. Other balloons includeuroplasty balloons for dilating the prostatic urethra.

The procedure illustrated in FIG. 2c can be facilitated by controllingthe location at which the biaxial orientation will occur. Anadvantageous manner of effecting this result is to carry out a localapplication of heat to the balloon portion 35 during radial expansionwhile avoiding such heat application at the leg portions 37. Elevatingthe temperature in such a localized area will lower the yield point ofthe nylon, polyamide or the like at that location and thereby facilitatethe biaxial orientation of this selected area. As an example, atemperature for conducting this step typically can be between about 35°C. and perhaps as high as 90° C., the optimum temperature dependingsomewhat upon the particular material that is utilized.

Means are provided for controlling the expansion or axial orientation ofthe tubing length 32a and its formation into the balloon portion 35 andthe legs 37. This can be accomplished by controlling and monitoringconditions and/or expansion positions within a molding apparatus. Anexemplary means in this regard is included in the molding apparatusdescribed herein. Otherwise, this function can be accomplished byclosely controlling the rate of fluid passage and by monitoring thepressure thereof. For example, for a valvuloplasty balloon, thefollowing approach can be taken. One end of the longitudinally orientedtube having a diameter of approximately 0.140 inch and a wall thicknessof approximately 0.006 inch is sealed, and a liquid such as water ispumped into the tube at a rate of approximately 2 ml per minute. Fortubing of this size and at a flow rate of this magnitude, ballooninflation begins at about 300 psi gauge and drops to around 150 psigauge. Expansion continues in this manner until the wall of a moldbearing the desired shape of the balloon is encountered, which isobservable by a significant pressure increase. Pumping continues until apressure of about 180 to 200 psi is reached. This pressure condition canbe held or maintained, if desired.

A satisfactory balloon can be prepared by proceeding with the methodthrough and including the step illustrated in FIG. 2c, followed bydimensional stabilization, heat setting or thermoforming, the balloon tonear its biaxially oriented profile and size by maintaining its elevatedtemperature until the selected material is thermally set. Tailorabilitythat is achieved according to this invention is a function of theparticular heat setting conditions. The setting temperature can varywith the type of material used, the wall thickness, and the treatmenttime. A typical setting temperature is between about 100° C. and about160° C. for a nylon material, and a typical time is from about 1 minuteto about 4 minutes.

Such balloons 33 may include expansion knurls 39 that tend to appear inthe areas generally extending between the legs 37 and the workingsurface of the balloon portion 35. These expansion knurls, which may bedescribed as nodules, ridges, ribs, beads or the like, are believed toresult from disproportionate expansion or orientation of the materialsuch as nylon in this area. When it is desired to minimize the existenceof these expansion knurls 39 on the balloon, secondary longitudinalorientation with radial shrinkage followed by secondary radial expansioncan proceed with a balloon that is not thermally set, such beingillustrated in FIG. 2d and FIG. 2e.

Regarding FIG. 2d, the balloon 33 of FIG. 2c, which is not thermallyset, is again longitudinally oriented by applying an axially directedforce that is typically greater than the yield point but less than theultimate tensile strength of the balloon 33. If desired, the magnitudeof this axial force can be substantially the same as the magnitude ofthe axial force applied in the procedure illustrated in FIG. 2b. Furtherbiaxial orientation is then conducted by, for example, introducing apressurized fluid into the balloon lumen in order to prepare the balloon34 shown in FIG. 2e. It is often desirable to conduct this procedurewithin a mold cavity in order to thereby effect a careful shaping of theballoon portion 36, the leg portions 38, and the tapered connectionsurfaces therebetween, generally as desired. The resulting modifiedballoon 34 typically will not include any significant expansion knurls39, and it will exhibit a uniformly oriented transition between theballoon portion 36 and the leg portions 38. This additional longitudinalexpansion or stretching substantially eliminates areas of non-orientedmaterial.

Whether the procedure is utilized that forms the balloon 33 or if theprocedure is continued such that the balloon 34 is formed, the thusformed balloon 33, 34 is preferably then subjected to a heat settingstep at which the still pressurized balloon 33, 34 is heated to set theexpanded dimensions thereof. Setting will typically be accomplished at atemperature of 85° C. or greater, typically up to about the meltingpoint of the balloon material, depending somewhat upon the actualmaterial and the size and thickness of the balloon. For example, atypical heat setting temperature for Nylon 12 is 1 minute at 120° C.With this heat treatment, the balloon will retain this form and most ofits expanded size, typically about 95% of more of it, upon being cooled.Preferably, the balloon remains pressurized until it is generallycooled. If this heat setting procedure is not utilized, the balloon willpromptly shrink back to approximately 40% to 50% of the diameter towhich it had been blown or biaxially oriented in the mold.

With more particular reference to this heat setting procedure, animportant component in this regard is the use of a thermoformablematerial. A material is thermoformed if it can be formed to anothershape. Nylon is a thermoforming plastic. It can be heated to atemperature below its melting point and deformed or formed to take onanother shape and/or size. When cooled, thermoforming materials retainthat new shape and/or size. This procedure is substantially repeatable.On the other hand, a thermosetting material, such as a natural latexrubber, a crosslinked polyethylene or a silicone rubber, once "set" iscrosslinked into that size and/or shape and cannot be heat deformed.Also, biaxially oriented polyethylene terephthalate tends to crystallizeand lose the thermoforming ability that is typical of polyethyleneterephthalate which is not biaxially oriented. When a biaxially orientednylon balloon is heated for a prolonged period of time at temperaturesgreater than a predetermined elevated temperature, for example about 80°C., and then cooled, it becomes thermoformed to the balloon geometry. Ifthe balloon were to be reheated to this temperature or above, it couldbe formed into a balloon having a different geometry.

FIG. 3 illustrates a molding apparatus that is suitable for fabricatingmedical balloons as discussed herein. The apparatus transforms a parison41 into balloons for medical devices and the like. The apparatusachieves longitudinal stretching, biaxial orientation, heating andcooling, and it includes means for monitoring radial expansion orbiaxial orientation, all of which can be conveniently controlled bysuitable means such as hard circuitry, a microprocessor, or othercomputerized controlling arrangements. These various parameters that arecontrolled can thus be precisely set and easily modified in order topresent the optimum conditions for fabricating a particular parison intoa balloon having a specified sizing and the properties desired. Specificparameter values are presented herein which are typically suitable forballoon materials such as nylons, and it will be understood that theseparameter values can be modified as needed for the specific materialbeing shaped into the balloon.

The apparatus that is illustrated also has the ability to generallysimultaneously perform different steps on multiple balloon portions ofthe parison 41 as it passes through the apparatus. This can beespecially useful because of variations in wall thickness and otherattributes of each batch of tubing that is used as the parison 41.

The apparatus includes a series of components which are suitably mountedwith respect to each other, such as along slide rails 42. From time totime, movement of some of the components along the rails or the like iscarried out by suitable means, such as piston assemblies (not shown) inaccordance with generally known principles and structures.

A pressurized fluid supply 43 is provided to direct pressurized fluidinto an end portion of the parison 41, typically in conjunction with oneor more regulators 44 in accordance with well-known principles. One ormore valves 45 also assist in controlling the flow rate and volume ofpressurized fluid within the parison 41. Gripper assemblies 46 and 47are provided for engaging different locations of the parison 41. Anintermediate gripper assembly 48 is preferably also provided. One ormore chiller chambers 51, 52, 53, 54 are also preferably provided. Theseare particularly useful as means for thermally isolating assemblies ofthe apparatus from each other. Also included are a free-blow biaxialorientation chamber or mold 55 and a molding chamber 56.

Preferably, means are provided by which the temperature of these variouschambers is controlled and/or varied, for example between about 0° C.and about 150° C. or more. The illustrated assembly in this regardincludes a jacket for confining a thermal fluid that is pumped thereintoand out thereof by suitable fluid inlets 57 and fluid outlets 58.O-rings 59 or the like are provided in order to contain the thermalfluid within each fluid jacket 61, 62, 63, 64, 65, 66.

In using the illustrated apparatus, the parison 41 is initially fed orthreaded through the entire apparatus, typically between gripperassembly 46 and gripper assembly 47. Gripping pads 68 of the gripperassembly 47 move inwardly and engage the downstream end portion of theparison 41 to the extent that it pinches off this portion of the parison41, preferably also heat sealing the parison at this time. Generallysimultaneously, gripping pads 67 of the gripper assembly 46 engage theparison 41 at the illustrated upstream location of the apparatus to suchan extent that the gripping pads 67 will prevent movement of the parison41 with respect to the gripping pads 67, but still permit the flow ofpressurized fluid thereacross when desired. Gripper assembly 47 thenmoves in a downstream direction (to the right as illustrated in FIG. 3)until the length of the parison that is secured between gripperassemblies 46 and 47 is stretched or longitudinally oriented to inexcess of twice its length up to as great as about four times its lengthor more, a typical stretching being approximately three times thisunstretched length.

Next, the free-blow biaxial orientation chamber 55 is heated by passingheated thermal fluid into the fluid jacket 62. The chiller chambers 51,52, or other suitable means, provide a thermal variant such that theheat from the fluid jacket 62 is imparted only to that length of theparison that is substantially within the free-blow biaxial orientationchamber 55. The temperature of this particular portion of the parison 41will be heated to a temperature of roughly between about 70° C. and 120°C. or more, depending upon the particular parison 41 and the balloonproperties desired. At this time, pressurized fluid within the parison41 that originates from the supply 43 passes through the parison lengthat the gripper assembly 46 and into the parison length at the free-blowchamber 55. If desired, the gripper assembly 48 can be utilized in orderto confine this particular pressure application to the section of theparison 41 that is upstream thereof.

A primary objective of the free-blow biaxial orientation chamber 55 isto radially expand that portion of the parison 41 into a balloon 33 asgenerally illustrated in FIG. 2c. It is often desired to preciselycontrol the amount of biaxial orientation, and means in this regard areprovided. The means illustrated in FIG. 3 includes a slidable insert orring 69. When this insert 69 is engaged by the expanding balloon 33, itmoves against an air or metallic spring (not shown) in a direction tothe left as illustrated in FIG. 3 until it trips a suitable switch orthe like (not shown), which signals that the desired degree of biaxialorientation has been achieved. At this time, steps are taken tointerrupt the biaxial orientation. This typically includes cooling byexchanging the heated thermal fluid within the fluid jacket 62 forthermal fluid having a colder temperature, typically on the order ofabout 10° C. or somewhat above or below depending upon the particularparison 41 and the particular properties desired.

The pressure imparted to balloon portion 33 is then depleted by, forexample, permitting exhaustion thereof through the valve 45 after theballoon has been cooled. If utilized, the gripper assembly 48 isreleased, and the balloon portion 43 is moved from the free-blow biaxialorientation chamber 55 into the molding chamber 56 by movement of thedownstream end portion of the parison by the gripper assembly 47. Thistypically simultaneously accomplishes the longitudinal orientation stagedepicted in FIG. 2d. Once this positioning takes place, the pressurizedfluid is pumped into that portion of the parison that is within themolding chamber 56, and thermal fluid is passed into the fluid jacket 65in order to heat this portion of the parison to an elevated temperature,again between about 70° C. and up to just below the melting point, forexample 150° C. or more, depending upon the particular parison and theproperties desired of the balloon.

Generally speaking, it is usually advantageous that the temperature inthe molding chamber 56 be higher than that applied in the free-blowchamber 55, while at the same time imparting a pressure to the insidewalls of the parison within the molding chamber 56 that is equal to orlower than the pressure applied in the free-blow chamber 55. Forexample, when a nylon is the material, the temperature in the free-blowchamber 55 can be slightly above ambient, preferably in a range ofbetween about 30° C. and 60° C., while the temperature in the moldingchamber 56 can be at the high end of this range or even well above, asneeded. Exemplary pressures would include on the order of tenatmospheres in the molding chamber 56 and twice that pressure or greaterin the free-blow biaxial orientation chamber 55. The exact pressure isdetermined by the material and by the wall thickness and hoop stress ofthe balloon to be molded.

With heat thus imparted to the modified biaxially oriented balloon 34within the molding chamber 56, the balloon 34 is thereby thermoformed,with heat setting in this regard involving raising the temperature ofthe thermoplastic while it is under inflated stress. Thereafter, theheated fluid within the fluid jacket 65 is exchanged for cooling fluidin order to substantially maintain the size and shape of the balloon 34formed within the molding chamber 56. After the pressure has beenrelieved, the balloon is removed from the apparatus. Subsequently, thethus modified parison is severed generally along lines A and B asillustrated in FIG. 2e in order to thereby form the balloon 26 forinclusion within a medical device such as the catheter 21.

It will be appreciated that the parison will include balloons in variousstages of their formation as the parison passes through the apparatus.Preferably, this is accomplished in a somewhat reverse manner, asfollows. After initial stretching and formation of a balloon 33 withinthe free-blow chamber 55, the apparatus is utilized so that the portionof the parison within the molding chamber 56 is radially expanded beforethat within the free-blow chamber 55, which generally occurs as follows.Molding chamber 56 is heated as described herein and pressurized to formthe balloon 34. Thereafter, the gripper assembly 48 closes off theparison between the chambers 55 and 56. The free-blow chamber 55 is thenheated, and the biaxial orientation is carried out as described hereinin order to form the balloon 33. Balloon 33 is then moved into themolding chamber 56, and the process is essentially repeated.

It should be appreciated that the balloon can be made entirely in themold section 56 without free blowing in compartment 55. However,balloons made in this manner may demonstrate knurling.

Regarding the fluids suitable for use in the apparatus, it is typicallyadvantageous to have the pressure source 43 provide a fluid that doesnot require excessive treatment to remove same from the internalsurfaces of the finished medical device balloon 26. To be taken intoconsideration in this regard are moisture content of fluids and ease andsafety of disposal of the fluid. A particularly suitable fluid ispressurized nitrogen gas. With respect to the fluid for use within thevarious fluid jackets 61 through 66, it is typically best to utilize afluid that is in its liquid state throughout whatever processingtemperatures might be needed for the apparatus. Preferably the fluid isone that will likewise maintain a generally consistent viscositythroughout the processing range, for example, avoiding the onset ofsolidification or crystal formation or the like that would modify theproperties of the thermal fluid at lower processing temperatures.

With respect to the materials out of which the balloon and parison aremade, they are typically nylons or polyamides. Preferably, thesematerials have an amorphous nature while still possessing enoughcrystallinity to be biaxially oriented under the conditions providedaccording to this invention. The materials should also have substantialtensile strength, be resistant to pin-holing even after folding andunfolding, and be generally scratch resistant. The material should havean intrinsic viscosity that enables blowing at elevated temperatures. Atypical intrinsic viscosity range that is preferred for the nylon orpolyamide materials according to this invention is between about 0.8 andabout 1.5, with about 1.3 being especially preferred. It is also desiredthat the material have a reasonably good moisture resistance. Materialsof the Nylon 12 type have been found to possess these advantageousproperties. Other exemplary nylons include Nylon 11, Nylon 9, Nylon 69and Nylon 66.

With more particular reference to the intrinsic viscosity of nylon orpolyamide materials, it is believed that such materials are able togenerally maintain their intrinsic viscosities during extrusion andthrough to final balloon fabrication. It is further understood thatother materials such as polyethylene terephthalate which have been usedto fabricate balloons for medical devices typically do not exhibit thismaintenance of intrinsic viscosity, but the intrinsic viscosity of thepellet material drops substantially during extrusion, with the resultthat a biaxially oriented polyethylene terephthalate balloon isunderstood to have an intrinsic viscosity which is lower than that ofthe polyethylene terephthalate pellets from which it originated. It isfurther believed that a higher balloon intrinsic viscosity reduces thelikelihood that pin-holing will develop in the finally fabricatedballoon.

With further reference to the nylon or polyamide materials which may beused according to this invention, they are bondable to the catheter 21by epoxy adhesives, urethane adhesives, cyanoacrylates, and otheradhesives suitable for bonding nylon or the like, as well as by hot meltbonding, ultrasonic welding, heat fusion and the like. Furthermore,these balloons may be attached to the catheter by mechanical means suchas swage locks, crimp fittings, threads and the like. Nylon or polyamidematerials can also be provided in the form in which they are reinforcedwith linear materials such as Kevlar and ultra high tensile strengthpolyethylenes. The polymer materials used can also be coated withpharmaceutical materials such as heparin and the like, non-thrombogeniclubricants such as polyvinyl pyrrolidone and the like. Additionally, thepolymer materials can be filled with radiopaque media such as bariumsulfate, bismuth subcarbonate, iodine containing molecules, tungsten, orother fillers such as plasticizers, extrusion lubricants, pigments,antioxidants and the like.

Nylons or polyamides as used according to the present invention areflexible enough to tolerate a greater wall thickness, even in the legareas, while still providing a structure that is foldable onto itselffor insertion into a body cavity or guiding catheter. These nylons orpolyamides, when biaxially oriented and formed into balloons asdiscussed herein, have a calculated tensile strength of between about15,000 and about 35,000 psi and above, preferably between about 20,000and about 32,000 psi.

The materials according to the present invention form medical deviceballoons that exhibit the ability to be expanded first to anon-stretched or non-distended condition, or working size, upon theapplication of a given pressure. They also have the ability to beinflated further so as to be stretched therebeyond in a controlled andlimited manner. The degree of such stretching or expansion can, withinlimits, be tailored as needed. In other words, the balloons according tothe present invention allow for some growth at a pressure higher thanthat needed to merely fill the balloon, but this additional growth orexpansion is not so substantial that there is an overinflation danger.

Materials according to this invention should be able to be tailoredduring balloon formation to possess the ability to be stretched agenerally predetermined percentage beyond its non-distended or workingdiameter, the amount of this percentage depending upon conditions underwhich the parison was processed into the balloon. A suitable materialaccording to the present invention can be tailored to cover valueswithin a span of at least 10 percentage points of radial expansion.Certain materials including some nylons can exhibit a tailorabilityrange of 25 or 30 percentage points or more and can, for example,exhibit a radial expansion of 10 percent or below and as high as about30 percent or above.

In order to illustrate this situation, tests were run on angioplastyballoons of different materials, namely polyvinyl chloride, cross-linkedpolyethylene, polyethylene terephthalate and nylon. These data arereported graphically in FIG. 4. Wall thicknesses varied depending uponthe properties of the material in order to provide a viable balloon. Thepolyethylene terephthalate balloon (B) had a wall thickness of 0.010 mm.The polyvinyl chloride balloon (C) had a wall thickness of 0.064 mm, andthe polyethylene balloon (D) had a wall thickness of 0.058 mm. (Data forballoons C and D are as reported in "Effects of Inflation Pressure onCoronary Angioplasty Balloons, " The American Journal of Cardiology,Jan. 1, 1986, these tests being run under ambient conditions, while theremaining balloons in FIG. 4 were tested at 37° C.) One of the nylonballoons (A) had a wall thickness of 0.013 mm and a hoop expansion ratioof 5.2. Another of the nylon balloons (E) had a wall thickness of 0.008mm and a hoop expansion ratio of 4.3. Tailorability of balloonsaccording to this invention is a function of hoop expansion ratio, whichis defined as mean balloon diameter divided by mean as-extruded tubingor parison diameter. Typically, such hoop expansion ratios range betweenabout 3 and about 6.

FIG. 4 plots the relative pressure of the fluid imparted to the variousangiographic catheters against the percentage increase in size (such asdiameter or radius) of the balloon beyond its working size, designatedas "0." It will be noted that the more compliant materials, such aspolyvinyl chloride, gradually grow with increased pressure and permitsome limited additional pressure increase after the working size hasbeen achieved and before further expansion ceases prior to reaching theburst limit of the balloon. It will be appreciated that, with materialssuch as these, a smaller increase (when compared with materials of plotsA and B) in relative pressure significantly increases the balloon size.Materials such as polyethylene terephthalate (PET) inflate to theirworking size, but not therebeyond to an extent as great as others of theplotted curves.

The nylon or polyamide material illustrated by curve E exhibits arelatively long and flat curve in FIG. 4, which indicates that it willcontinue to expand moderately upon the application of relatively smalladditional increments of inflation pressure. Once the unstretched ornon-distended profile or working size is reached, this tailored nylonballoon possesses adequate stretchability or compliance to the extentthat it can ramp up to another inflated profile. While the polyethyleneballoon of curve D initially distends to about the same extent as doesthe nylon curve E balloon, this nylon balloon can tolerate greaterinflation pressures without bursting than can the curve D polyethyleneballoon or the curve C polyvinyl chloride balloon. This observation isespecially significant when it is appreciated that the curve E nylonballoon had a substantially thinner wall diameter. Based upon data suchas that illustrated in FIG. 4, it is possible to predict the compliantexpansion associated with added inflation pressure. A balloon inaccordance with the present invention can exhibit this growth propertyso that it will radially expand from a low of about 5 percent beyond theworking size to a high of about 35 percent or more of the working size.

As previously observed herein, balloon expansion tailorability is afunction of heat setting conditions and of hoop expansion ratio forballoon materials according to the present invention. For suchmaterials, increasing the heat set temperature increases the size of thefinished balloon at a given inflation pressure and decreases theshrinkage which typically occurs after sterilization such as a standardethylene oxide treatment. Also, the amount of hoop expansion imparted tothe parison during processing has a significant effect on the amount ofdistention which the finished balloon will exhibit.

FIGS. 5 and 6 illustrate the effect of varying heat setting conditionson the distensibility of the finished balloon. This heat setting isachieved when the biaxially oriented balloon is subjected to atemperature greater than the glass transition temperature of the balloonmaterial and is allowed to cool to below the glass transitiontemperature. For a material such as a nylon or a polyamide, temperaturesbetween about 90° C. and about 180° C. have a pronounced heat settingcapability. The glass transition temperatures may be less than this heatsetting temperature and may include temperatures in the range of 20° C.to about 40° C., and the balloon will exhibit good flexibility at bodytemperature which can, for example, facilitate balloon insertion andmaneuverability during use.

FIG. 5 gives four distention plots, tested at 37° C. for nylon balloonsthat are substantially identical except they were subjected to differentheat set temperatures. In each case, the balloon was subjected to a 2minute heat/cool cycle, with cooling being to room temperature. Eachballoon was subsequently subjected to ethylene oxide sterilization.Curve F was subjected to a heat set temperature of 120° C., curve G wasat 130° C., curve H was at 140° C., and curve I was at 150° C. It willbe noted that the curves illustrate different distention properties andthe type of tailoring thus achieved. FIG. 6 provides a similarillustration and further shows an effect of the ethylene oxidesterilization. While curves J and K both are for balloons which wereheat set at 100° C., the curve J balloon was sterilized while the curveK balloon was not. Likewise, the balloons of curves L and M both wereheat set at 140° C., curve L illustrating a sterilized situation, andcurve M a non-sterilized situation.

The relationship between balloon tailorability and hoop expansion ratiois illustrated in FIG. 7. Three non-sterilized nylon balloons havingdifferent hoop expansion ratios were tested at 37° C., the expansionbeing to burst. A relatively high hoop expansion ratio of 4.9 (curve N)gave a balloon distention (safely short of burst) of about 7 percent. A3.7 hoop expansion ratio (curve O) gave a balloon distention of about 23percent, while a 3.3 hoop expansion ratio (curve P) gave a balloondistention of about 36 percent.

Furthermore, nylon or polyamide balloons according to the presentinvention provide controlled expansion properties and tolerate givenpressures without bursting without requiring wall thicknesses as greatas those needed for other materials such as polyvinyl chloride orpolyethylene. This thin wall thickness of these nylon balloons permitsthe balloon to enter smaller vessels, lesions and/or catheters. Suitableballoons according to this invention can have shell or wall thicknessesas low as about 0.0002 inch and as high as 0.002 inch. An exemplarypreferred shell thickness range is between about 0.0004 and about 0.001inch, or between about 0.0005 and about 0.0008 inch.

A material such as nylon is softer and more flexible at bodytemperature, the temperature at which it is used, than other balloonmaterials such as polyethylene terephthalate which have a glasstransition temperature far in excess of body temperature. For at leastsome nylons, the glass transition temperature approximates bodytemperature, and nylon balloons generally exhibit amorphous propertiesthat enhance flexibility and softness even after proceeding with thetype of processing described herein, including thermoforming and thelike. Nylon balloons thus can be at a glass transition state while theyare within the body, which is not the case for numerous other materialsthat can be used as medical balloons.

Balloons made of nylons exhibit a property such that, when they burst,the diameter of the balloon will decrease, thereby facilitating removalfrom the body. It is believed this decrease may be caused by a relaxingof the material which noticeably reduces the size of the balloon. Othermaterials such as polyethylene terephthalate are not known to exhibitthis advantageous property. A disadvantageous property that can becharacteristic of polyethylene terephthalate medical balloons is thedevelopment of extensive material wrinkling upon sterilization, whichwrinkles persist, even at body temperature, until very high inflationpressures are applied, on the order of 200 psi. When a nylon orpolyamide balloon is sterilized, few if any material wrinkles develop,and these substantially disappear at relatively low inflation pressures,on the order of 8 psi.

Nylon materials have been observed to exhibit desirable stability duringprocessing to the extent that they do not absorb excessive moisture fromthe environment if the parison is allowed to stand uncovered forreasonable time periods. Materials such as polyethylene terephthalateare degraded by environment moisture if the parison is not stored inprotective bags or the like prior to biaxial orientation.

It will be understood that the embodiments of the present inventionwhich have been described are illustrative of some of the applicationsof the principles of the present invention. Numerous modifications maybe made by those skilled in the art without departing from the truespirit and scope of the invention.

We claim:
 1. A process for tailoring expansion properties of a balloonfor a medical device, the process comprising the steps of:longitudinallystretching a length of biaxially orientable tubing having an initialdiameter and made of material capable of being tailored by the stepshereof to provide drawn tubing; radially expanding the thus drawn tubingto form a balloon member, said balloon member having a non-distendedworking diameter and having a hoop expansion ratio, which hoop expansionratio is an approximate ratio of said non-distended working diameter tosaid initial diameter of the tubing; and said radially expanding stepincluding selecting said hoop expansion ratio such that the balloonmember exhibits a tailored maximum inflated diameter to which theballoon stretches without bursting during dilatation, said hoopexpansion ratio being selected in accordance with the fact that saidmaximum inflated diameter increases as said hoop expansion ratiodecreases.
 2. A process according to claim 1, wherein said hoopexpansion ratio is between about 3 and about
 6. 3. The process accordingto claim 1, further including heat setting said balloon member byraising the temperature of the balloon member to a heat settingtemperature above the glass transition temperature of the biaxiallyorientable tubing material, followed by cooling the balloon member to atemperature below the glass transition temperature, said heat settingtemperature being selected in accordance with the fact that said maximuminflated diameter increases as said heat setting temperature increases.4. The process according to claim 3, wherein said heat settingtemperature is between about 90° C. and about 180° C.
 5. The processaccording to claim 3, wherein said glass transition temperatureapproximates human body temperature.
 6. The process accordance to claim3, wherein said temperature below the glass transition temperatureapproximates room temperature.
 7. The process according to claim 3,wherein said heat setting step maintains said heat setting temperaturefor between about 1 minute and about 4 minutes.